This invention relates to compandors, for example, a circuit for providing DC bias control of a compressor.
Companding is a widely used technique for gaining a signal to noise ratio improvement in systems where a signal is passed through a noisy transmission medium. Companding is utilized when one wishes to transmit a signal with a large dynamic range through a channel that has limited dynamic range. The process of companding is a method whereby data with a large dynamic range is first "compressed" thereby attenuating the high voltage signals and amplifying the low voltage signals. This compressed limited dynamic range signal is then usually transmitted over a channel. Upon receipt, the data is "expanded" thereby amplifying the high voltage signals and attenuating the low voltage signals.
The basic building blocks of a compandor are an operational amplifier (op amp), a rectification and average circuit, and a variable gain stage. The op amp is typically connected in a negative feedback mode whereby the rectification and average circuit and the variable gain stage are configured in the feedback loop for a compressor or connected to the inverting input of the op amp for an expander.
The rectification and average circuit performs rectification of the input signal, for the expander, or of the output signal, for the compressor, and then averages the rectified signal to obtain a DC signal that is proportional to the average level of the input or output signal for a expander or compressor, respectively. This DC signal then feeds the variable gain stage which determines the overall gain of the compandor. Additionally, the gains of a compressor and an expander are typically complimentary wherein the output of the former is a function of the square root of the input signal, while the output of the latter is a function of the square of the input signal.
With respect to a compressor, a variable resistor is utilized in the feedback path and the variable resistor is typically infinite when no input signal is present. As a result, a separate DC feedback path is required to properly bias the compressor in the absence of an input signal.
One approach to bias the compressor when there is no input signal present is described in an article by Craig C. Todd entitled "A Monolithic Analog Compandor", IEEE JSSC vol. sc-11. In particular, the subject compandor includes a separate DC feedback path comprised of external resistors and an external capacitor. However, in addition to requiring these three additional components, two additional pins are also required which may not be suitable on an 8-pin integrated circuit or any pin limited application.
Hence, a need exists for a circuit that provides DC bias control of a compressor in the absence of an input signal which minimizes the number of external components and pins required.